1. Field of the Invention
The present invention relates to an ink set capable of forming an image excellent in, in particular, light fastness and ozone resistance, and excellent in preservation stability. The invention further relates to an ink cartridge, inkjet printer, inkjet recording method and recorded matter using the ink set.
2. Background Art
The inkjet recording has rapidly spread in recent years and is still continuing development for the reasons that the materials are inexpensive, high speed recording is possible, noise during recording is little, and color recording is easy.
The inkjet recording method includes a continuous method of continuously jetting droplets, and an on-demand method of jetting droplets according to image information signal. Ejecting methods of droplets include a method of ejecting droplets by the application of pressure by means of a piezo element, a method of ejecting droplets by generating bubbles in ink by heating, a method of using ultrasonic wave, and a method of suction/ejection of droplets by electrostatic force.
As the ink for inkjet recording, aqueous inks, oil inks and solid (melting type) inks are used.
Such dyestuffs used in the inkjet recording ink are required to be excellent in solubility or dispersibility in a solvent, to be capable of high density recording, to have good hue, to be fast to light, heat, and active gases in the environment (NOx, oxidizing gases such as ozone, and SOx), to have excellent resistance to water and chemicals, to have good fixing ability to image-receiving materials and hardly bleed, to be excellent in preservation stability as the ink, to be nontoxic, to have high purity, and to be inexpensively available.
For example, bis-azo dyestuffs showing good hue and having high fastness to light, ozone, and wet heat are disclosed in Japanese Patent No. 4402917 as yellow dyes.
Further, in recent years, obtaining recorded matters by making color images by an inkjet recording method using plural color ink compositions is performed. Color images are generally formed with three colors of a yellow ink composition, a magenta ink composition, and a cyan ink composition, and further, if necessary, four colors by adding a black ink composition thereto. In addition, there are cases where color images are formed with six colors including the above four colors plus a light cyan ink composition and a light magenta ink composition, and further, with seven colors of the above six colors plus a dark yellow ink composition. Those obtained by combination of two or more kinds of ink compositions as above are ink sets (for example, refer to JP-A-2007-138124).
It is required of the ink sets for use in the formation of color images as described above that the ink composition itself of every color has a good coloring property and, in addition, a good intermediate color can be developed when plural ink compositions are used in combination, and the obtained recorded matters are not discolored or decolored during preservation.
Further, when the light fastness and ozone resistance of a specific ink composition are conspicuously lower than the light fastness and ozone resistance of other ink composition in an ink set, the color formed by the specific ink composition is discolored or decolored faster than other colors, and the tone balance of the image at large worsens. As a result, observers come to be capable of recognizing deterioration of picture quality of the image in a shorter time than the time to be capable of recognizing discoloration of an image formed with a single ink composition.
Accordingly, in an ink set, in addition to the improvement of light fastness and ozone resistance of each ink composition constituting the ink set, it is preferred that light fastness and ozone resistance levels of each ink composition, i.e., deterioration speeds due to light and ozone of an image formed with each ink composition, for example, discoloration speed, is equal as far as possible. That is to say, it is preferred that light fastness and ozone resistance of each ink composition constituting the ink set are excellent and, at the same time, the difference in light fastness and ozone resistance between each ink composition is small.
An image having various color densities can be obtained by constituting an ink set including two kinds of dark and light ink compositions of the same system of colors different in color density, so that an image free of the feeling of graininess has come to be capable of being obtained. Such an ink set containing two kinds of ink compositions different in color density is mainly used in printing of a photographic image. In the formation of a photographic image, for relieving or solving the feeling of graininess of an image, an ink composition of low color density is generally used in many cases. Also in evaluation of the above-described light fastness and ozone resistance, a pattern of the evaluation sample of optical density in the vicinity of 1.0 is formed with an ink composition of low color density. Accordingly, for the purpose of the improvement of light fastness and ozone resistance of the photographic image of the recorded matter and light fastness and ozone resistance as ink set at large, it is preferred to improve light fastness and ozone resistance of ink composition of low color density.
On the other hand, since high color density ink compositions are used in printing of an image of brilliant colors and a graphic art pattern, it is also necessary to improve light fastness and ozone resistance of these ink compositions.
Further, a black ink composition performs an important role in an image from the viewpoint of obtaining the contrast of an image. Accordingly, a black ink composition is contained in an ink set in many cases.
Therefore, when a black ink composition is contained in an ink set, it is desired that the black ink composition has excellent light fastness and ozone resistance and also the speed of deterioration of the black ink composition due to light and ozone is not conspicuously different from the speeds of deterioration of other ink compositions constituting the ink set.